1. Field of the Invention
The present invention relates to an improved susceptor. More particularly, the invention relates to a susceptor which inhibits the deposition of process gases on the edge and back side of a substrate.
2. Background of the Related Art
Chemical vapor deposition (CVD) is one of a number of processes used to deposit thin films of material on semiconductor substrates. To process substrates using CVD, a vacuum chamber is provided with a susceptor configured to receive a substrate. In a typical CVD chamber, the substrate is placed into and removed from the chamber by a robot blade and is supported by the susceptor during processing. A precursor gas is charged to the vacuum chamber through a gas manifold plate situated above the substrate, where the substrate is heated to process temperatures, generally in the range of about 250.degree.-650.degree. C. The precursor gas reacts on the heated substrate surface to deposit a thin layer thereon and to form volatile by-product gases, which are pumped away through the chamber exhaust system.
A primary goal of substrate processing is to obtain the largest useful surface area possible from each substrate. This is highlighted by the recent demands from semiconductor chip manufacturers for zero edge exclusion on the substrates processed. Some important factors to consider include processing variables that effect the uniformity and thickness of the layer deposited on the substrate and contaminants that can attach to the substrate and render all or a portion of the substrate useless. Both of these factors should be controlled to maximize the useful surface area for each substrate processed.
One cause of particulate contamination in the chamber is deposition of material at the edge or on the backside of the substrate. Substrate edges are typically beveled, making deposition difficult to control over these surfaces, thus deposition at substrate edges can be non-uniform. This may lead to deposited layers that do not adhere properly to the substrate edge and eventually chip or flake off, causing unwanted particle generation in the chamber.
Additionally, chemical mechanical polishing is often used to smooth the surface of a substrate coated with tungsten or other metals. The act of polishing will cause any deposits on the edge and backside surfaces to flake off and generate unwanted particles.
Thus, different approaches have been employed to control the deposition of process gases on the edge of a substrate during processing. One approach employs a shadow ring that essentially masks a portion of the perimeter of the substrate from the process gases. This reduces the overall useful surface area of the substrate and in light of the current demand from chip manufacturers for zero edge exclusion, this method is becoming impractical.
Another approach employs a gas manifold near the edge of the substrate for the delivery of purge gas past the edge to prevent edge deposition on the substrate as shown in U.S. Pat. No. 5,556,476 to Lei et al. The purge gas inhibits the deposition of process gases at the substrate edge, but the purge gas also mixes with the process gas and is typically exhausted through the same manifold as the process gas. This mixing can lead to dilution of the process gas and/or non-uniform deposition of the gases on the substrate surface.
A third approach uses a shadow ring and a purge gas channel in combination to form a purge gas chamber adjacent to the substrate edge having a purge gas inlet and outlet. This system requires a higher pressure within the purge gas chamber than in the process chamber to keep the process gas from being drawn into the purge gas outlet. Therefore, the purge gas is drawn into the chamber and out through the processing chamber exhaust system. Drawing the purge gas into the exhaust system of the process chamber can have a negative effect on the process uniformity over the substrate surface.
Therefore, there is a need for a device that allows for full surface coverage of a substrate which prevents backside and edge deposition without disrupting the uniformity of the process gases and the resulting uniformity of the film formed on the substrate.